BG recommended the use of MarcEdit to handle the binary Marc file, and it works a treat; downloaded the Linux-y distro, which is actually just the .NET version, but with some extras, and after installing the mono-complete package, this runs with mono MarcEdit.exe. It read the large binary file and was able to convert it into a larger XML file, which is a bit big for Oxygen to handle comfortably, but can be browsed, and will certainly be amenable to XQuerying in eXist.